Dispersion strengthened silver

ABSTRACT

The invention provides a fully dense compacted metal composite comprising silver alloy matrix having uniformly dispersed therein discrete microparticles of a refractory metal oxide, and further comprising discrete macroparticles of metal oxides. This composition can be used for electrical contacts which have greater mechanical strength, welding resistance and arc erosion resistance than previously known electrical contacts.

BACKGROUND OF THE INVENTION

[0001] Dispersion strengthened metals are well known. For example, dispersion strengthened copper (hereinafter called “DSC”) is produced by forming an alloy of copper as a matrix metal and aluminum as a refractory oxide forming solute metal. The alloy containing from 0.01% to 5% by weight of the solute metal, is comminuted by atomization, or by conventional size reduction methods to a particle size, desirably less than about 300 microns, preferably from 5 to 150 microns, then mixed with an oxidant. The resultant alloy powder-oxidant mixture is then compacted prior to heat treatment, or heated to a temperature sufficient to decompose the oxidant to yield oxygen to internally oxidize the solute metal to the refractory metal oxide in situ and thereby provide a very fine and uniform dispersion of refractory oxide, e.g., alumina, throughout the matrix metal. Thereafter the dispersion strengthened metal is collected as a powder or submitted to size reduction to yield a powder having a particle size of from less than 850 microns to submicron size for use herein. Mechanical alloying of the matrix and solute metals as by prolonged ball milling for a powder mixture for 40 to 100 hours can also be used prior to internal oxidation.

[0002] Dispersion strengthening can be accomplished in a sealed can or container. The alloy powder may be recrystallized prior to dispersion strengthening.

[0003] Silver can be dispersion strengthened by the method described above or a variation thereof. Silver has high electrical conductivity but lacks mechanical strength. Internal oxidation of a solute metal to form a refractory oxide in the matrix metal (silver) substantially increases the mechanical strength of silver. Alloys or composites of silver are used as electrical contacts, and in other general electrical conductor applications. It is common to add metal oxides. such as CdO, SnO₂, ZnO, etc., to silver to provide resistance to welding of the contacts to each other during the make and break cycles. The oxides also partially sublime or evaporate, and thus provide an arc quenching or evaporative cooling effect. However these oxides do not increase the strength or hardness of the contact material significantly. The dispersion strengthened silver matrix of this invention increases the strength and hardness of the contact material significantly. The refractory oxide dispersion improves the arc erosion resistance and provides some resistance to welding of the contact. Metal oxides, such as SnO₂, ZnO, Co₃O₄, MoO₃, WO₃, etc. are added to further improve the resistance to welding and also provide an arc quenching capability. This invention relates to materials for use in making electrical contacts for medium and low power electrical equipment, processes for making the materials, and contacts made from the material.

SUMMARY OF THE INVENTION

[0004] The invention provides a fully dense compacted metal composite comprising silver or silver alloy matrix having uniformly dispersed therein discrete microparticles of a refractory metal oxide, and further comprising discrete macroparticles of metal oxides. This composition can be used for electrical contacts which have greater mechanical strength, welding resistance and arc erosion resistance than previously known electrical contacts.

DETAILED DESCRIPTION OF THE INVENTION

[0005] This invention uses a refractory metal oxide dispersion strengthened silver or silver alloy matrix as the base material. The oxide in the dispersion strengthened silver can be any of the stable refractory oxides such as Al₂O₃, ZrO₂, ThO₂, Y₂O₃, HfO₂, MgO, SiO₂, TiO₂, etc. Al₂O₃ is preferred because of the high solid solubility of aluminum in silver or silver alloy which helps maintain all the aluminum in solid solution, thus avoiding the segregation of aluminum. The amount of Al₂O₃ may vary from 0.2% to 1.1%, the equivalent aluminum content is from 0.1% to 0.6%. The solid solution alloy is atomized into powder and the aluminum is internally oxidized in situ. The refractory metal oxide is very uniformly dispersed by virtue of internal oxidation of the solute metal. In this resulting base material is dispersed a mixture of discrete macroparticles selected from the group consisting of tin oxide (SnO₂), cobalt oxide (Co₃O₄), molybdenum oxide (MoO₃), tungsten oxide (WO₃) and zinc oxide (ZnO). The macroparticles range in size from about 0.1 micron to about 100 microns. The addition of the macroparticles decreases the welding of electrical contacts. The amounts of oxides could vary from 1 to 15% by weight. These oxides partially sublime or evaporate at elevated temperatures, and thus are expected to provide an arc quenching effect.

[0006] Ag—CdO type electrical contacts can be made by adding a suitable oxide to the Ag—Al₂O₃ base material. Materials shown in Examples 3 and 4 have used Co₃O₄, SnO₂ and ZnO to replace the toxic CdO. These oxides partially sublime or evaporate at elevated temperatures, and thus are expected to provide a similar arc quenching effect as that of CdO. The Al₂O₃ dispersion in the base material provides some anti-welding properties, and the use of Co₃O₄, SnO₂, ZnO, etc. adds to this capability. These oxides also have lubricating properties which will reduce the wear due to adhesion. The amounts of oxides vary from 1 to 15% by weight. The CdO in the conventional contacts does not provide much improvement in hardness while the Al₂O₃ dispersion in the materials of this invention increases the hardness of the matrix and reduce the wear and arc erosion.

EXAMPLES Example 1

[0007] The following example is to the base dispersion strengthened silver. A solid solution alloy with Ag+0.25% Al was melted and atomized by high pressure nitrogen. The powder thus produced was heated in air at 1600° F. to oxidize the Al in situ to form Al₂O₃. The internally oxidized powder was then pressed into 4 inch diameter cylinders and enclosed in a steel container. The container was evacuated, sealed and hot extruded into a 3 inches wide×0.5 inch thick bar. A small piece of this bar was used for evaluation. The steel can was removed and the bar was milled down to 0.25 inch thickness. A section of this bar was used for testing and the remainder was cold rolled into two different strip thicknesses. Properties of the as rolled strip and after annealing at 1200° F. were measured. Table-1 shows the data on all the materials. TABLE 1 Cold Electrical Work Hardness 0.2% YS UTS Elong. Conductivity Material Condition (%) (R_(B)) (ksi) (ksi) (%) (% IACS) 3″ × Extruded  0 46 36 47 16 85 0.25″ 3″ × Rolled 81 — 64 76  8 85 0.048″ 3″ × Annealed 81 — 52 61 13 85 0.04 8″ 3″ × Rolled 97 — 95 107  8 85 0.008″ 3″ × Annealed 79 — 79 88 10 85 0.008″

[0008] This material is an example of the base material to which the metal oxide is added.

Example 2

[0009] A solid solution alloy containing Ag+0.27% Al was melted and atomized under high pressure nitrogen. Two different materials were prepared as follows:

[0010] Material A: A 300 g sample of this powder was blended with 13 g of a fine Co₃O₄ powder. The blend was heated at 1600° F. in nitrogen to oxidize the Al in situ to form Al₂O₃. After internal oxidation, an estimated 4% of CoO was left in the material as a residue from the Co₃O₄.

[0011] Material B: Another sample of the same powder was heated in air as described in Example 1 to internally oxidize the Al in the alloy.

[0012] Both Material A and B were enclosed in copper cans and hot extruded into 0.500 inch×0.125 inch bars. The copper can was removed from all sides and the bars were tested. Table-2 shows the results. TABLE 2 Electrical Hardness 0.2% YS UTS Elong. Conductivity Material (R_(B)) (ksi) (ksi) (%) (% IACS) A 63 44 49 4 69 B 73 54 63 12 74

Example 3

[0013] Solid solution alloys containing Ag+0.27% Al and Ag+0.55% Al were melted and atomized under high pressure nitrogen. The powders were internally oxidized as described in Example-1. These were labeled C & D respectively. Samples of each powder were then blended with 10% SnO₂, 10% ZnO and 10% Co₃O₄. They were labeled as follows:

[0014] Material C1: C+10% SnO₂

[0015] Material C2: C+10% ZnO

[0016] Material C3: C+10% Co₃O₄

[0017] Material D1: D+10% SnO₂

[0018] Material D2: D+10% ZnO

[0019] Material D3: D+10% Co₃O₄

[0020] All these powders were enclosed in copper cans and hot extruded into 0.500 inch×0.125 inch bars. The copper can was removed from all sides and the bars were tested. Table-3 shows the results. Also shown for comparison are the properties of an industry standard Ag+10% CdO material made by hot extrusion process and used in electrical contacts. This data is from ASM Handbook, Vol. 7, Page 1023. TABLE 3 Electrical Hardness 0.2% YS UTS Elong. Conductivity Material (R_(B)) (ksi) (ksi) (%) (% IACS) C 73 54 63 12 58 C1 78 53 62 2 48 C2 74 50 58 2 50 C3 76 49 60 1 46 D 82 66 74 10 51 D1 80 56 66 2 40 D2 79 54 64 1 42 D3 80 59 71 1 41 Ag + 10% 46 R_(F) (<0 R_(B)) — 25 — 84-87 CdO* Ag + 10% 40 — 33 — 84-87 CdO**

[0021] The materials produced in this experiment have much superior mechanical properties.

Example 4

[0022] A solid solution alloy containing Ag+0.25% Al was melted and atomized under high pressure nitrogen. The powder was internally oxidized as described in Example 1 and labeled E. Samples of this powder were blended with 5% and 10% SnO₂, ZnO and Co₃O₄. These powders were labeled as follows:

[0023] E1: E+5% SnO₂

[0024] E2: E+5%ZnO

[0025] E3: E+5% Co₃O₄

[0026] E4: E+10% SnO₂

[0027] E5: E+10% ZnO

[0028] E6: E+10% Co₃O₄

[0029] 20 g samples of each of the powders were pressed into 20 mm diameter. discs to approximately 85-87% of the theoretical densities of the respective materials. These were sintered at 1650° F. in nitrogen and then repressed to 95-99% of the respective theoretical densities. These were then tested for hardness and electrical conductivity. The results are shown in Table-4. Properties of Ag+5% CdO and Ag+10% CdO materials made by the Press-Sinter-Repress (PSR) process are also shown for comparison. This data is from ASM Handbook, Volume 7, Page 1023. TABLE 4 Hardness Electrical Conductivity Material (R_(B)) (% IACS) E 64 73 E1 67 53 E2 70 56 E3 68 61 E4 58 41 E5 72 44 E6 68 47 Ag + 5% CdO 32 R_(F) (<0 R_(B)) 80-90 Ag + 10% CdO 42 R_(F) (<0 R_(B)) 72-85

[0030] Here again the hardness levels in the materials of this invention are much higher than those of conventional Ag+CdO materials. These materials are expected to offer superior arc erosion resistance in electrical contacts.

Example 5

[0031] A quantity of the internally oxidized Ag+0.25% Al powder from Example 1 was labeled F. Following blends were made from this as the base powder:

[0032] F1: F+1%SnO₂

[0033] F2: F+2% SnO₂

[0034] F3: F+5% SnO2

[0035] The base powder and the above blends were pressed into 1.15 inch diameter cylinders to about 80% of the theoretical densities for the respective materials. These were enclosed in steel cans, evacuated and sealed. The billets thus produced were hot pressed to consolidate the cylinders to near full theoretical density for the respective materials. The steel can was removed and the billets were then hot extruded into 0.25 inch diameter. rods. These rods were cold drawn to 0.080 inch diameter. wires except that material F3 could not be drawn to this size. Some intermediameterte annealing steps were required. Table-5 shows the properties of the wires. Also shown for comparison are the data on an Ag+2.5% CdO material. This data is from ASM Handbook, Volume 7, Page 1023. TABLE 5 0.20% YS UTS Material Condition (ksi) (ksi) F, 0.080″ diameter. As Drawn 63 71 F, 0.080″ diameter. Annealed, 1200° F. 50 59 F1, 0.080″ diameter. As Drawn 70 76 F1, 0.080″ diameter. Annealed, 1200° F. 54 62 F2, 0.080″ diameter. As Drawn 68 72 F2, 0.080″ diameter. Annealed, 1200° F. 57 62 Ag + 2.5% CdO Cold Worked — 25 Ag + 2.5% CdO Annealed — 19

[0036] The materials of this invention again show much higher strength levels than the commercial Ag+CdO material at similar oxide levels.

[0037] Other composites and alloys of the base material would be possible depending on the requirements of the specific applications. 

What is claimed:
 1. A fully dense compacted powdered silver composite comprising (a) a silver or silver alloy matrix having uniformly dispersed therein discrete microparticles of a refractory metal oxide and (b) discrete macroparticles of a metal oxide selected from the group consisting of tin oxide (SnO₂), cobalt oxide (Co₃O₄), molybdenum oxide (MO₃), tungsten oxide (WO₃) and zinc oxide (ZnO).
 2. The metal composite of claim 1 wherein the refractory metal oxide is aluminum oxide.
 3. The metal composite of claim 1 wherein the metal oxides are selected from the group of tin oxide, cobalt oxide, molybdenum oxide, tungsten oxide and zinc oxide.
 4. The metal composite of claim 2 wherein the concentration of aluminum oxide in the matrix is in the weight range of from 0.2% to about 1.1%
 5. The metal composite of claim 1 wherein the concentration of the metal oxide is in the weight range of from 1% to 15%.
 6. A method of making a dispersion strengthened silver composite containing macroparticles of a metal oxide comprising the steps of: a. providing a powdered metal alloy of silver and a refractory oxide forming solute metal; b. internally oxidizing said solute metal by heating said alloy in air or in the presence of a suitable oxidant; c. combining the oxidized alloy with macroparticles of a metal oxide; d. thermally coalescing said internally oxidized alloy and said metal oxide into dispersion strengthened composite stock.
 7. Electrical contacts made by using the composition of claim
 1. 